The present invention relates to a breathing circuit having an improved means for removing water vapor from the circuit. Condensation of the water vapor within the breathing circuit, and its attendant problems, is thus lessened or eliminated. The invention is particularly suited for use in breathing circuits characterized as being of the closed type.
During breathing, a volume of breathing gas, termed the tidal volume, is inhaled into the lungs during inspiration and exhaled during expiration. Tidal volumes typically range from 400-1000 milliliters (ml), depending on the size of the subject""s lungs. When in the lungs, the breathing gases become moistened and water vapor is discharged with the breathing gases when they are expired. About 35 milligrams (mg) of water are discharged by a person with each breath, assuming a tidal volume of about 1000 milliliters.
A mechanical ventilator may be used to supply and remove breathing gases to/from the subject. This may be done to assist or replace the natural breathing action of the subject, in connection with the supply of an anaesthetic agent to the subject, or for other reasons. A typical mechanical ventilator has an inspiration limb for supplying breathing gases to the subject and an expiration limb for receiving breathing gases from the subject. The inspiration and expiration limbs are each connected to arms of a Y-connector. A patient limb extends from a third arm of the Y-connector to an intubation tube or face mask for the subject.
A common type of mechanical ventilator recirculates the expired breathing gases of the subject in the expiration limb through a CO2 absorber back to the inspiration limb for rebreating by the subject. Such a closed breathing circuit prevents loss of anaesthetic agents to the ambient air. Such breathing circuits are often operated in a xe2x80x9clow flowxe2x80x9d mode in which, at least in principle, the amount of fresh, dry breathing gases added to the breathing circuit is only that necessary to replace the gases consumed by the subject.
However, the CO2 absorber in such a circuit acts as a moisture reservoir so that additional moisture, for example, an additional 15 mg of water per breath becomes entrained in the breathing gases circulating in the closed breathing circuit.
While it is preferable that the subject breath moist, warm breathing gases, the presence of such gases in the breathing circuit does have disadvantages. When the warm, moist breathing gases expired by the subject, which are at body temperature, pass through the breathing circuit, which is at room temperature, the water vapor in the breathing gases condenses on components of the breathing circuit. As the breathing of the subject continues, the condensed water accumulates. The accumulated water may interfere with the operation of valves, sensors, or other components of the breathing circuit or form a medium for microbiological growth within the circuit. Such accumulations therefore present a problem in closed circuit breathing systems.
Various solutions have been proposed to remedy this problem. Water traps may be inserted in the breathing circuit in an effort to prevent water from reaching critical components. Or, all, or the portions, of the breathing circuit particularly effected by moisture accumulation, may be heated to prevent condensation of the water vapor. This may be carried out for example by resistance heaters, such as wires that are wrapped around the tubing of the limbs, and around valves, etc.
However, while heating can delay the onset of condensation and prevent condensation in critical parts of the circuit, it is difficult or impossible to fully prevent precipitation of water vapor out of the breathing gases.
Many breathing circuits incorporate a humidity and moisture exchanger (HME) in the patient limb in which heat and moisture from exhaled breathing gases are exchanged to the breathing gases to be inhaled by the subject. The primary purpose of such an exchanger is to provide for patient comfort by preheating the inhaled breathing gases and to ensure that the patient does not inhale dry breathing gas. However, in the usual case, not all moisture in the exhaled breathing gases is transferred to the inhaled breathing gases. A small amount, which can be characterized as xe2x80x9cleakagexe2x80x9d remains in the exhaled gases and circulates in the breathing circuit. Thus, the reduction in the moisture level of the exhaled gases entering the breathing circuit, as a result of humidifying the inhaled gases, may also delay, but usually will not fully prevent, condensation and moisture accumulation in other portions of the breathing circuit.
Due to the additional amount of water inserted in the breathing gases by the CO2 absorber, the amount of breathing gas water vapor is increased in the portions of the breathing circuit between the absorber and the subject, exacerbating the problems of moisture condensation on sensors, traps, and the like in this portion of the circuit.
The problem is particularly acute when a ventilator is operated in a low-flow manner since the breathing gases are continually recirculating and little fresh, dry gas is being added.
The present invention is thus directed to a breathing circuit having an improved means for removing water vapor from the breathing gases in the breathing circuit. Condensation within the breathing circuit and its attendant problems is thereby lessened or eliminated, including that occurring under low flow conditions.
Briefly, the breathing circuit of the present invention incorporates a dryer downstream of the CO2 absorber for removing water vapor from the breathing gases. The dryer may incorporate means, such as a thermoelectric element, for cooling breathing gases passing through the absorber so that the water vapor condenses out of the breathing gases. A fan may be used in conjunction with the thermoelectric element to improve its performance. The dryer may include a heat transfer means for reheating the cooled, drier breathing gases with incoming gases from the CO2 absorber.
Or, the breathing gases may be passed along a water permeable membrane such as a water permeable tube to remove the water vapor from the breathing gases. The tube may be jacketed so that dry air can be supplied to the other side of the tube to improve the removal of the water vapor.